HDLC Frame types n n n Information frames, or I-frames, transport user data from the network layer. In addition they can also include flow and error control information piggybacked on data. Supervisory Frames, or S-frames, are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames do not have information fields. Unnumbered frames, or U-frames, are used for various miscellaneous purposes, including link management. Some U-frames contain an information field, depending on the type. http: //en. wikipedia. org/wiki/High-Level_ Data_Link_Control#I-Frames_. 28 user_data. 29 21

802. 3 MAC of Ethernet (CSMA/CD) n CSMA/CD: 1. If the medium is idle, transmit; otherwise, go to step 2 2. If the medium is busy, continue listening (CS: carrier sensing) until the channel is idle, then transmit immediately 3. If a collision is detected (CD) during transmission, transmit brief jamming signal to assure all stations know about collision and then cease transmission 4. After transmitting the jamming signal, wait a random time (back-off time), then attempt to transmit again 35

Basic and Extended Service Sets n Basic Service Set (BSS) – tens of meters Operate in Basic Service Area (BSA) that is much like the area of cell in mobile communications n BSSs may geographically overlap, be physically disjoint, or they may be collocated (one BSS may use several antennas) n Ad-hoc or Infrastructure (nomadic) mode: Access coordinated by the given instance of MAC Extended Service Set (ESS) n Multiple BSSs interconnected by Distribution System (DS) n Each BSS is like a cell and stations in BSS communicate with an Access Point (AP). n Portals attached to DS provide gateways to access Internet or other ESS n n 62

IEEE 802. 11 Mobility n n Standard defines the following mobility types: n No-transition: no movement or moving within a local BSS n BSS-transition: station movies from one BSS in one ESS to another BSS within the same ESS n ESS-transition: station moves from a BSS in one ESS to a BSS in a different ESS (continuos roaming not supported) Especially: 802. 11 don’t support roaming with GSM or 3 G! - Address to destination mapping - seamless integration of multiple BSS ESS 2 ESS 1 68

802. 3 Ethernet PHY r 10 Mb DIX Ethernet uses baseband transmission, that is, the adapter sends a digital signal directly into the broadcast channel. r The interface card does not shift the signal into another frequency band, as do ADSL and cable modem systems. DIX Ethernet (10 Mb/s) uses Manchester encoding (next slide) r With Manchester encoding each bit contains a transition; a 1 has a transition from up to down, whereas a zero has a transition from down to up. r The reason for Manchester encoding is that the clocks in the sending and receiving adapters are not perfectly synchronized. By including a transition in the middle of each bit, the receiving host can synchronize its clock to that of the sending host.

Manchester encoding r used in 10 Base. T (DIX Ethernet) r each bit has a transition r allows clocks in sending and receiving nodes to synchronize to each other m no need for a centralized, global clock among nodes! Ref: Kurose, Ross: Computer Networking 5: Data. Link Layer 5 -90